cmake: create a relative symlink to clang, not absolute

Hardcoding the location of the build directory seems like it would
break at `make install`/packaging time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150201 91177308-0d34-0410-b5e6-96231b3b80d8

tools/driver/CMakeLists.txt

@@ -40,16 +40,17 @@ set_target_properties(clang PROPERTIES VERSION ${CLANG_EXECUTABLE_VERSION})
 
 if(UNIX)
   set(CLANGXX_LINK_OR_COPY create_symlink)
+# Create a relative symlink
+  set(clang_binary "clang${CMAKE_EXECUTABLE_SUFFIX}")
 else()
   set(CLANGXX_LINK_OR_COPY copy)
+  set(clang_binary "${LLVM_BINARY_DIR}/bin/${CMAKE_CFG_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX}")
 endif()
 
 # Create the clang++ symlink in the build directory.
 set(clang_pp "${LLVM_BINARY_DIR}/bin/${CMAKE_CFG_INTDIR}/clang++${CMAKE_EXECUTABLE_SUFFIX}")
 add_custom_command(TARGET clang POST_BUILD
-    COMMAND ${CMAKE_COMMAND} -E ${CLANGXX_LINK_OR_COPY}
-    "${LLVM_BINARY_DIR}/bin/${CMAKE_CFG_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX}"
-    "${clang_pp}")
+    COMMAND ${CMAKE_COMMAND} -E ${CLANGXX_LINK_OR_COPY} "${clang_binary}" "${clang_pp}")
 
 set_property(DIRECTORY APPEND
   PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${clang_pp})